1. Field of the invention
The present invention relates to an image display apparatus such as an automatic scanning type display monitor capable to be synchronized with various synchronizing signals having various frequencies, and more particularly to a character generating circuit in such an image display apparatus for generating characters to be superimposed on the image displayed on the image display apparatus.
2. Description or the Related Art
FIG. 1 is a diagram showing in detail a display character generating unit 4 in a conventional image display apparatus. In the figure, reference numeral 41 represents a display character generating IC, 42 is a clock signal generating unit for generating a display character generating clock signal C' which is fed into the display character generating IC 41, and H and V are a horizontal synchronizing signal and a vertical synchronizing signal. The display character generating clock signal is hereinafter referred to as a basic clock signal.
A description will now be given of the operation. In order to carry out character display in synchronization with an input signal inputted into the image display device, the display character generating IC 41 is fed with the horizontal synchronizing signal H and the vertical synchronizing signal V which are included in the input signal. The display character generating IC 41 is controlled in response to a control signal CC.
On the other hand, the clock signal generating unit 42 generates the basic clock signal C' necessary for generating characters to be displayed. The basic clock signal C' fed from the clock signal generating unit 42 is identical with a dot clock signal for a pixel forming a display character.
Here, the display character generating IC 41 will be described in brief. The display character generating IC 41 is provided with a built-in character generator ROM in which character data such as alphanumeric characters are previously stored. Therefore, it is possible to display a desired character by designating an address in the character generator ROM in which the character is stored.
On the other hand, as shown in FIG. 2A, in the character data stored in the character generator ROM, each character is formed by 12.times.18 dots (width.times.height), each dot consisting of d.sub.V in vertical width and d.sub.H in horizontal width. For example, "01" shown in the drawing means an address in which a character data "1" is stored. That is, it is possible to display the character data "1" by addressing the storage address "01." The character to be displayed is designated by the control signal CC in FIG. 1. The control signal CC is supplied from a control unit (corresponding to a control unit shown in FIG. 5 as described later) which controls the entire image display apparatus.
A character data designated to be displayed by the control signal CC is read from the ROM for generating characters, dot by dot, in synchronization with the basic clock signal C' outputted from the clock signal generating unit 42. Further, the character data is outputted from the display character generating IC 41 as display character data. FIG. 2B shows this state. In the drawing, f.sub.C, is the frequency of the dot clock signal C', t.sub.C' is a period which is expressed as t.sub.C' =1/f.sub.C' and corresponds to the length d.sub.H of one dot.
Here, the dot clock signal is in synchronization with the horizontal synchronizing signal H shown in FIG. 1 inputted into the display character generating IC 41.
It is assumed that the frequency f.sub.C' of the dot clock signal C' is a constant frequency f.sub.C. When, for example, the frequency of the horizontal synchronizing signal H inputted into the display character generating IC 41 is f.sub.H, the maximum number n.sub.H of characters which can be horizontally displayed in one horizontal period is given as follows: ##EQU1## (where a horizontal blanking period is not taken into account as a matter of convenience.)
FIG. 3A, FIG. 3B, and FIG. 3C show a timing relationship between the display character and a horizontal periodic signal. As shown in FIG. 3A, characters "CONTRAST MAX" or "BRIGHT 00", for example, are displayed in one horizontal synchronization period. When the frequency of the basic clock signal is increased, the period for displaying one character is shortened, as shown in FIG. 3B and FIG. 3C.
A description will now be given of a case in which the frequency f.sub.H of the horizontal synchronizing signal H inputted into the display character generating IC 41 increases.
In this case, it is assumed that the frequency of the horizontal synchronizing signal H is f.sub.H'. Then, since the frequency f.sub.C' of the dot clock signal C' is the constant frequency f.sub.C, the maximum number n.sub.H of characters which can be horizontally displayed in one horizontal period can be expressed as follows: ##EQU2##
By comparing the equations (1) and (2), since f.sub.H &lt;f.sub.H' as assumed above, the relation n.sub.H &gt;n.sub.H' can be obtained. That is, when the frequency f.sub.H of the horizontal synchronizing signal H increases, the number of characters which can be horizontally displayed is decreased.
For example, if f.sub.H' =2f.sub.H, the following expressions can be derived from equations (1) and (2): E1 ? ? ##STR1##
That is, when the frequency f.sub.C' of the dot clock signal C' is the constant frequency f.sub.C, and when the frequency of the horizontal synchronizing signal H is doubled, the number of characters which can be displayed in one horizontal synchronization period is reduced by one half (see FIG. 4A and FIG. 4B).
The conventional image display apparatus is provided as set forth above. Consequently, there is a problem in that, when the frequency of the horizontal synchronizing signal H in an input signal increases, the size of the displayed character becomes large so that the number of characters which can be displayed on the display becomes smaller than the necessary number of characters to be displayed.